The invention relates to a multiband antenna.
Most mobile communication is handled via the GSM 900 network, that is to say in the 900 MHz band. In addition, the GSM 1800 Standard has been established, inter alia, in Europe, in which Standard signals can be transmitted and received in an 1800 MHz band.
Such multiband base stations therefore require multiband antenna devices for transmitting and receiving different frequency bands, which normally have dipole structures, that is to say a dipole antenna device for transmitting and receiving the 900 MHz band range and a further dipole antenna device for transmitting and receiving the 1800 MHz band range.
In practice, therefore, multiband, or at least two-band, antenna devices have already been proposed, namely, for example, a dipole antenna device for transmitting the 900 MHz band and for transmitting the 1800 MHz band, with the two dipole antenna devices being arranged alongside one another. Two antennas are therefore required in each case for the at least two frequency band ranges which, in fact, since they are arranged physically alongside one another, interfere with one another and have an adverse effect on one another, since they shadow each other""s polar diagram. It is thus no longer possible to achieve an omnidirectional polar diagram.
It has therefore also already been proposed for two corresponding antenna devices to be arranged one above the other for operation in two different frequency band ranges. This, of course, leads to a greater physical height and demands a larger amount of space. In addition, the omnidirectional polar diagram is in some circumstances also adversely affected, at least to a minor extent, since the connecting line leading to the higher antenna device has to be routed past the lower antenna device.
The object of the present invention, in contrast, is to provide an improved two-band or multiband antenna device.
According to the invention, this object is achieved by the features specified in claim 1. Advantageous refinements of the invention are specified in the dependent claims.
In comparison to the prior art, the present invention provides, in a surprising manner, a completely novel, extremely compact antenna device which can be operated in a two frequency band range. However, if required, this antenna device can also be extended as required for a multiband range covering more than two frequency bands.
Specifically, the invention provides for the dipole antenna device for the first frequency band and the dipole device for the at least second frequency band, which is offset from the former, to be formed coaxially with respect to one another and in the process, such that they are located interleaved in one another.
To this end, according to the invention, the dipole halves are preferably in the form of sleeves, with the sleeve diameters of the dipole halves differing from one another to such an extent that the sleeves are arranged one inside the other. The length of the dipole halves in this case depends on the frequency band range to be transmitted. Those dipole halves which are in the form of sleeves, are designed to have the shorter length and are required for the higher frequency band range are in this case located on the outside, with those dipole halves which are designed to be appropriately longer for the lower frequency band range being arranged inside these outer sleeves, with their length projecting beyond the outer dipole sleeves.
The outer and inner sleeves of the dipole halves are each electrically and mechanically connected at their inner ends to a short-circuiting point which is similar to a sleeve base, with the one dipole halves, which are interleaved in one another in the form of sleeves, making contact with an inner conductor, and the other dipole halves, which are interleaved in one another, making contact with the outer conductor.
The particular feature of this design principle is that, for example, the outermost dipole halves which are in the form of sleeves and are suitable for the higher frequency band range act as dipole radiating elements towards the outside, but act as a detuning sleeve towards the inside, so that those dipole halves which are in the form of sleeves and are provided for the low frequency band range cannot be identified for these radiating elements.
Those dipole halves which are in the form of sleeves, are provided for the lower frequency band range and, in contrast, are each designed to be longer act as radiating elements over their entire length outwards, without the blocking effect of the outer radiating element, which is in the form of a sleeve, having any effect for the higher frequency band range, but act as a detuning sleeve towards the inside, so that no surface waves can propagate onto the outer conductor.
If more than two frequencies or frequency bands are to be transmitted, the design principle can be extended appropriately, with the sleeves for the higher frequency each having a larger diameter in their shorter length extent, and the dipole halves, which are in the form of sleeves, for the lower frequency band range in each case being accommodated such that they are interleaved in one another.
This design principle also allows central feeding via a common connection or a common coaxial line, which is preferably used not only for feeding but is also used at the same time for mechanical robustness and holding the antenna. The coaxial vertical tube which is in the form of the outer conductor is in this case mechanically and electrically connected to the one dipole half at the appropriate feed point, that is to say at the short-circuiting point of this dipole half, with the inner conductor continuing slightly beyond the outer conductor, where it is electrically and mechanically attached to the short-circuiting points, which are similar to sleeve bases, of the other dipole halves. If the inner conductor has appropriate strength, there is no need for any further additional measures for robustness. Otherwise, additional measures which electrically have no effect but are used for robustness could be provided between the short-circuiting points, which are in the form of sleeves, of the mutually adjacent dipole halves. Apart from this, the entire antenna illustrated in the attached figure is accommodated in a protective tube, for example a tube composed of glass-fiber-reinforced plastic, which engages over the antenna arrangement, fitting it as accurately as possible, so that the inner conductor has to withstand and absorb only the weight of the upper dipole halves, since tilting loads and movements are absorbed by the protective tube.
It can also be seen from the figure that a further major advantage is that only a single coaxial cable connection is required for feeding the at least two or more frequency band ranges to the antenna device.
However, the dipole halves need not necessarily be in the form of tubular structures which are in the form of sleeves and are short-circuited at their feed points. These dipole halves, which are in the form of sleeves, may have circular or cylindrical cross sections, or may be provided with a polygonal or even oval cross section. They need not necessarily be in the form of closed tubes, either. Multi-element structures are also feasible, in which the dipole halves, which are similar to sleeves, are composed of a number of individual conductor sections or electrically conductive elements, or are broken down into these sections or elements, provided these sections or elements are short-circuited to one another at their respective feed end which is adjoined to the respective adjacent second dipole.
In particular, according to the invention, not only a single band but also a multi-frequency band antenna device is possible, which preferably comprises at least two antenna devices located one above the other, which can in turn transmit in at least two frequency band ranges each.
This can be achieved according to the invention in that the coaxial feed line arrangement is routed axially through that antenna device which is preferably in each case lower, and is continued to the next higher antenna device. In the feed line, the outer electrical conductors of the multiple coaxial feed lines are in each case used to feed the dipole halves of the lower antenna device while, in contrast, those conductors of the coaxial line (for example the inner conductor, which is generally in the form of a wire, and the innermost coaxial conductor surrounding it) which are inside the former are in each case used for electrically feeding that antenna device which is higher than the other and has the dipole halves provided there.
The design principle can be cascaded in a corresponding manner, so that three or more antenna devices can also be arranged one above the other.
This can preferably be achieved in a highly advantageous and effective manner by using a specific feed and output-coupling apparatus.